Solid oxide fuel cells (SOFCs) generate electricity through the spontaneous transport of an oxygen ion across a strong chemical potential gradient. The chemical potential gradient for oxygen is maintained by the renewal of air on the cathode side, and the replenishment of fuel on the anode side. While there has been much interest in the use of hydrogen as the fuel, hydrocarbon fuels are still the most practical as they have the highest energy density and commercial availability.
SOFCs operate at high temperatures. Hydrocarbon fuels, however, have a propensity to coke, or form carbon deposits, at high temperatures, whether by gas phase pyrolysis or through heterogeneous catalytic routes. To avoid carbon formation, the fuel is prepared by reforming it to more refractory molecules such as methane, carbon monoxide and hydrogen, as well as by removing the thermodynamic driving force for solid carbon. Carbon activity, which is the thermodynamic propensity for carbon formation, is particularly sensitive to the presence of oxygen, and as such, for high temperature fuel cells, it is generally advisable to maintain an oxygen atom to carbon atom ratio of approximately 1:1 to 2:1 in the fuel entering the fuel cell. Mitigation of carbon deposition in high temperature fuel cells can be achieved by lowering of the carbon activity through the addition of an oxidant in the fuel stream, typically either air or water.
Reformers for fuel cells have been developed that are located external to the fuel cell. External steam reformers are generally large and complex and require a separate source of water. Some external reformers utilize oxygen present in the hot anode exhaust as carbon dioxide and water. For example, the hot anode exhaust is collected in a manifold outside the fuel cell stack and recirculated by high temperature blowers or ejectors back into the fuel cells. Ejector systems have a more simple design compared to hot gas blower systems, although their efficiency falls off rapidly as the pressure ratio increases (or orifice size decreases), and high pressure (˜100 psi) is required to generate the high suctions necessary for overcoming the pressure head of the stack.